1. Field of the Invention
The present invention relates generally to getter materials, and more particularly to multilayer deposits including an over layer of low activation temperature getter alloy.
2. Description of the Related Art
Non-evaporable getter materials, also known in the art as NEG materials, include transition metals such as Zr, Ti, Nb, Ta, V, and alloys or compounds thereof with one or more elements selected from Cr, Mn, Fe, Co, Ni, Al, Y, La and other rare earth elements. Such alloys include binary alloys such as Ti—V, Zr—Al, Zr—V, Zr—Fe and Zr—Ni, ternary alloys such as Zr—V—Fe and Zr—Co-rare earth elements, and other multi-component alloys. They can also include metal compounds (e.g. metal oxides), non-metal, organics, etc. NEG materials are scavengers, removing selected species, typically gaseous, depending upon their composition and operating conditions.
NEG materials are, for example, capable of reversibly sorbing hydrogen and irreversibly sorbing gases such as oxygen, water, carbon oxides and, in some cases, nitrogen. These materials are used for maintaining vacuum, as in, for example, evacuated interspaces for thermal insulation. Getter materials are also used to remove the above-mentioned species from inert gases, primarily noble gases and nitrogen, for example in gas-filled lamps or in the manufacture of ultrapure gases such as used in the microelectronics industry.
NEG materials can be employed in various forms, such as, for example, sintered pills or powders of the material within suitable containers. In some applications, for reasons of available space or for simplicity of construction, NEG materials are provided in the form of thin layers, generally tens or hundreds of microns (μm) in thickness, on an inner surface of an apparatus. Examples of uses of thin layers of NEG material are disclosed in U.S. Pat. No. 5,453,659, which describes Field Emission Displays (known in the art as FEDs), wherein discrete and thin deposits of NEG material are formed among electron-emitting cathodes on the anodic plate of a display. U.S. Pat. No. 6,468,043 describes coating of the inner surface of pipes defining the chamber of a particle accelerator with a NEG layer. U.S. Pat. Nos. 5,701,008 and 6,499,354 describe, respectively, the use of getter materials in micromechanical devices and in miniaturized IR radiation detectors. Micromechanical or microoptoelectronic devices are known in the art as “micromachines” or MEMs (microelectromechanics). In all of these applications, the NEG deposit (after activation) is employed at room temperature.
The functioning of NEG materials is based on reaction between the NEG metal atoms and the above-mentioned gaseous species. As a result of such reaction, oxide, nitride and/or carbide species are formed on the NEG surface at room temperature, resulting eventually in the formation of a passivating layer that prevents further gas sorbing. This passivating layer can form rapidly in the presence of large amounts of gas, for example at the first exposure to the atmosphere of the freshly produced NEG material, or during certain “dirty” manufacturing steps of the devices in which the material is contained. The layer forms, although more slowly, over time as a result of the normal functioning of the NEG in sorbing gaseous species.
At the beginning of its operating life, a NEG typically undergoes a thermal activation treatment, normally under vacuum, whose object is the migration of passivating layer species towards the inside of the material structure, thereby exposing a fresh and active metallic surface for gas sorption. The activation may be complete, providing, for example, a material surface essentially entirely made up of metal, or partial, providing, for example, a “mixed” surface, made up of areas of oxide-type species (or the like) and metallic areas. An activation degree can be defined, corresponding to the fraction of “free” surface sites, i.e. metals in the elemental state and consequently available for reaction with gases. In some cases, the activation treatment can be periodically repeated during a device's operating life, in a process called reactivation, to restore the initial NEG gas sorbing properties.
In theory, complete activation of an NEG would generally be desirable, but it can be unfeasible in the manufacture of certain devices, due to restrictions on manufacturing times and/or heat sensitivity of the particular device. Accordingly, partial activation is used for such devices, even though this results in lower gas sorbing properties and shorter NEG operating life.
The level of activation is dependent on process temperature and time; for example, an activation degree of 70% of a given material can be reached by treatment for 30 minutes at 350° C. or for 10 hours at 250° C., with the effect of temperature being greater than that of time. The conditions of activation also vary according to the physicochemical characteristics of the given material. For some materials, complete activation can require very high temperatures. For example, complete activation of an 84:16 Zr:Al alloy requires temperatures of at least 700° C. and preferably about 900° C., unless extremely long times, typically unacceptable in industrial production, are used. Other alloys, such as some ternary Zr—V—Fe alloys, require much lower activation temperatures and can be completely activated at about 350° C. in about one hour. As used herein, a “low activation temperature” material refers to a material (metal, intermetallic compound or alloy) which can be activated to a high degree (e.g. about at least 90%) by a treatment of one hour at a maximum required temperature of 300° C.
NEG deposits made up of a single metal (and particularly those of titanium, which are the most commonly used) can be easily manufactured by sputtering with open or porous morphology, which increases the effective surface area and consequently the initial gas sorption rate. For an effective activation (or reactivation), however, pure metals require comparatively high temperatures, generally higher than 450° C. In miniaturized devices such as FEDs or MEMs, wherein the NEG material is quite close to functional or structural parts of the device, the activation treatment can damage these parts. For example, in the case of FEDs, in which the NEG is generally placed at the peripheric region, heating at 400° C. can compromise the tightness of sealing between the two glass parts forming the display, which are made up of a low-melting glass paste. Similarly, exposure to these temperatures can compromise sealings between silicon components of MEMs, which are often composed of brazing alloys such as silver-based alloys or gold-tin or gold-indium alloys.
Certain NEG intermetallic compounds or alloys have low activation temperatures, for example about 300° C. or lower. However, the present inventors have determined that these materials, when deposited by sputtering, give rise to thin layers having extremely compact morphology and consequently a very reduced effective surface area, generally equivalent to only a few times the deposit geometrical area. This characteristic limits considerably the deposit sorbing properties at room temperature, particularly its initial sorption rate and its capacity. Such materials would require frequent reactivation, which may be impractical or impossible in certain applications.
Accordingly, currently known NEG deposits obtained by sputtering either have poor sorbing characteristics at room temperature (in particular, a low sorption rate) or require high activation temperatures incompatible with some applications, particularly in miniaturized devices. It would therefore be desirable to provide NEG materials which are characterized by low activation temperature and a large surface area.